Magnetic amplifier



Patented Mar. 15, 1949 UNITED STATES PATENT OFFICE MAGNETIC AMPLIFIER Application April 13, 1945, Serial No. 588,194

19 Claims. 1

This invention relates to magnetic amplifiers of the type which employ saturating reactors and has for an object the provision of an amplifier in which the amplified output current not only follows the input signal but is reduced to zero with a zero input signal and which reverses its polarity with reversal of polarity of the input signal. It is a further object of the invention to provide methods of and means for magnetically and linearly amplifying currents of low order of magnitude while preventing deleterious effects which would otherwise arise when the input current greatly exceeds the normal range.

Magnetic amplifiers, heretofore known to the art, and of the type disclosed in my patents, Nos. 2,027,312 and 2,277,849, have been satisfactory for a number of applications but their usefulness has been limited to some extent, particularly for applications where the input signal may substan tially exceed its normal range. a magnetic amplifier is designed for a given set of conditions as regards the input and output thereof. As long as the input signal remains within the normal range as determined by the design of the system, the amplifier functions quite satisfactorily. When the input signal materially exceeds its normal range, magnetic amplifiers are either paralyzed or they are subject to hysteresis, the result of which is that their performance subsequent to the application of an unduly high signal differs from their performance before application of such unduly high signal.

By the expression paralyzed there is meant that the magnetic conditions which are prerequisite to amplification of the input signal are so disturbed as to produce an output signal which bears little or no relation to the input signal. For example, as the input signal increases materially above its normal range, the output current will still increase but at a rate lower than that of the input signal. For still greater input signals the output current will remain substantially constant and for even higher input signals the output current will actually decrease as the input signal increases.

The manner in which the hysteresis effect is manifested depends upon the type of magnetic amplifier involved. In arrangements in which the output of the amplifier energizes a relay such as in my Patent No. 2,027,311, the relation between the pick-up and drop-out of the relay is affected. In a magnetic amplifier of the type shown in the present application, in which the input and output is normally zero, the hysteresis causes a zero In other words,

shift; that is, when the input signal is zero the output signal may not be zero, or vice versa.

It is an object of the present invention to provide a magnetic amplifier which will withstand input signals of magnitude far beyond the normal range for which the amplifier has been designed, while still giving linear amplification within the normal range.

In polarity-responsive magnetic amplifiers of the prior art, as exemplified in my Patent No. 2,277,849, balanced outputs have been utilized; that is, for zero input signals, output currents are balanced. For signals of one polarity, the output currents are unbalanced in one direction and for signals of opposite polarity the output currents are unbalanced in the opposite direction. These output currents have been applied to electromechanical devices of the type responsive to differing electrical magnitudes. Such amplifiers do not lend themselves to cascade operation.

It is, therefore, a further object of the present invention to provide an amplifier of any desired number of stages arranged in cascade and whose output currents will reverse as the polarity of the input signals reverses.

In carrying out the present invention in one form thereof, there is provided in the currentresponsive input circuit a resistor connected in series therewith and which is also connected in shunt relationship to the output circuit. This resistor, for input currents exceeding the normal range, produces a non-linear opposition. to current flow in the input circuit.

So long as the input current is below a predetermined magnitude there is negligible opposition to current flow. Therefore, this feature does not decrease the sensitivity; that is, it does not increase the threshold response; it does not require input signals of increased magnitude. But for inputs, currents, above the predetermined magnitude the opposition rapidly increases. Thus, there is provided a current blocking-limiter which avoids application to the saturating reactors of signals whose magnitudes may produce a zero shift or deleteriously affect their desired operation.

Further in accordance with the invention, there may be provided a second resistor in series with the input circuit and in series with an output circuit which linearly varies the opposition offered to current flow in the input circuit.

For a more complete understanding of the invention and for further objects and advantages thereof, reference should be had to the accompanying description taken in conjunction with the drawing, in which:

Fig. l is a wiring dlagramdiagrammatically illustrating one form of the invention;

Fig. 2 is a graph explanatory of the operation of one of the devices shown in the system of Fig. l; and

Fig. 3 diagrammatically illustrates the magnetic circuit of one of the saturating reactors of Fig. 1.

Referring to the drawing, the invention in one form has been shown as applied to the amplification of input signals from a condition-responsive device I which may be of any suitable character,

more particularly one providing an output cur--- is subjected to the temperature under measurement and the other of whose junctions is connected to a potentiometer set in accordance with the desired temperature.-- The deviation from the desired temperature, whether it be in one direction or the other, produces a current of very low magnitude and of polarity or direction of flow dependent upon the sign of the deviation. In such an application the amplifier must respond to changes in inputs of the order of 50 microvolts. On the other hand, it must be capable of withstanding, without deleterious efl'ects, inputs as high as 50 millivolts. For example, operation at low temperature of a high temperature thermocouple with the potentiometer set for the high temperature control-point might produce as much as 50 millivolts.

' The current from the device H3 is applied to input windings H and I2 of a pair of saturating reactors l3 and M which are included in the first stage of the amplifier. The first stage of the amplifier consists of the elements illustrated below the broken line l5 while the elements appearing above-the broken line l5 comprise the second stage of the amplifier. The second stage also includes saturating reactors Il and 18. Each reactor, such for example as the reactor l3, Fig. 3, includes an input winding It, a bias winding 20, a positive feedback winding 2i, and a pair of coils 22 and 23 forming an alternating current winding. The windings ll, 25 and 26 are so arranged on the magnetic structure that no voltage is induced in them by the alternating cur-= rent flux produced by coils 22 and 23. The direction of the magnetomotive forces produced by these coils is shown by the arrows. The reactor I4 is similarly provided with a bias winding 25, a positive feedback winding 25, and an alternating current winding comprising coils 2'! and 25. The alternating current windings 2223 and 21-28 are respectively energized from secondary windings 30 and 3| of a transformer 32 whose primary winding 33 is connected to a suitable source of alternating current supply, having a frequency of 60 cycles per second, and where conveniently available a frequency above seven hundred cycles per second and below fifteen hundred cycles per second.

Since the saturating reactors I3, l4, l1 and I8 are structurally similar, only one of them, the reactor l3, will be described in detail. Referring to Fig. 3, it will be observed the windings ll, 29 and 2| are wound around the central leg of a magnetic 'core 29. The flux produced by the resultant magnetomotive forces developed by the windings ll, 20 and 2! passes outwardly from the central leg and divides between the endportions of the magnetic structure 23. The coils 22 and 23 of the alternating current winding are respectively wound on the end-portions of the magnetic structure or core 29.

It will be observed, Fig. 1, that the winding 22-23 is connected in series-circuit relation with the input terminals of the full-wave rectifier 3B. The output terminals thereof are connected in a circuit which includes the feedback winding 2| and the interstage resistor 35. Similarly, the coils 2i and 28 of reactor id are connected in series with the input terminals of a fullwave rectifier 35 while the output terminals of the rectifier are connected in series with the feedback winding 26 and the interstage resistor 31. The bias windings 20 and 25 are energized from the secondary winding 38 of transformer 32 through a full-wave rectifier 39. The circuit may be traced from the output terminals of the rectifier 39 by way of the bias winding 20, a resistor 40, the bias winding 25, resistor 4 l, and by conductor 42 to the other terminal of the rectifier 39. The magnetomotive forces of the windings 20 and 25 respectively oppose the magnetomotive forces of the windings 2i and 25. The value of resistor 45, in series with rectifier 39, is selected so that the current in windings 20 and 25 is such as to produce Operation on the desired portion of the magnetization curve, preferably in the region where maximum'permeability occurs. For ease in adjusting the bias produced by the windings 20 and 25, an output zero-adjusting resistor 48 is connected in shunt with them. A movable contact B9 is arranged to increase the resistance in shunt with .one winding while decreasing the resistance in shunt with the other winding, or vice versa.

Since the windings 26 and 25, of only a few turns, have a very low resistance, the resistance of resistors and 4! is suitably selected in relation to the resistance of resistor 48 in order to permit the latter to exercise the current-balancing operation which is desired for zero-adjustment. In the absence of a signal from the device Iii, the contact 49 is moved to a position such that the rectified current flowing through the coupling resistors 35 and 31 produces equal and opposite potential differences thereacross.

It will be assumed the device It has a polarity such as to produce current flow through the conductor 50, the input winding ll, thence by conductor 5i, resistors 52 and 53, conductor 54, input winding I2, resistor 55, and by conductor 56 to the other side of the device Ill. The input current in winding it controls the reactor l3 so as to increase the current flowing through the revsistor 35 while the input current in winding l2 controls the reactor I l so as to decrease the current flowing through the resistor 37. The result is that the potential difference across the resistor 35 materially increases and that across resistor 31 materially decreases, with consequent application to conductors to and 6! of an amplified potential difference, the conductor 60 being positive with respect to conductor 5!.

More specifically, the increased magnetomotive force resulting from the current flowing from the device I0 through the input winding it produces a decreased impedance of windings 22-23,, with consequent increase of alternating current through them. This increased current, after, rec tification by rectifier 34, flows through winding 2| which produces an increase in its magnetomotive force in a direction to still further increase the currents in windings 22-23. and interstage resistor 35. Thus, the winding 2| produces a positive feedback, or regenerative action.

Concurrently, the magnetomotive force resulting from the current from device I; ilowing through the winding l2, produces an increased impedance of windings 21-28, with consequent decrease of the alternating current through them. This decreased current, after rectification by rectifier l6, flows through winding 2! which then produces less magnetomotive force thereby further decreases the currents in windings 21-54! and interstage resistor 31.

Though any number of additional stages of amplification may be provided, only one has been illustrated. Thus, the amplified outputcurrent from the first stage flows by way of conductor through the input winding 62 of reactor", thence by conductor 63 to the input winding N of the reactor l8, and returns by way of conductor ii to the first stage. The remaining elements oi the second stage of the amplifier correspond with those of the first stage, and these have already been described in detail. The action is such that the current delivered to the second stage is similarly and regeneratively amplified and in turn is delivered to an output circuit including conductors 5 and 56. That the cascading of stages is an important achievement will be understood by realizin that the present invention has been applied to systems where the input current may be of the order of one microampere amperes) and input power of the order of one micro micro-watt (10- watts).

Further in accordance with the invention, approximate proportionality between the input current from device [0 and the output current may be secured. This is accomplished by flow of both the input and output currents through a common resistor and in the same direction. More specifically, the output current from the second or last stage of the amplifier flows through an ammeter 61, a resistor 68 and through the resistor 55. It flows through resistor 55 in the same direction as the current from device I. to produce a limiting efiect on the current flowing in'the input circuit. If desired. the resisto'r' i! may be connected in series with the output circuit Hi. of the first stage of the magnetic amplifier although the connection in the last stage of the amplifier is at present preferred.

As the magnitude of the input current increase the magnitude of the amplified output current flowin in resistor 55 likewise increases. The potential drop across resistor 55 opposes the potential of the device l0, thereby decreasing the current flowing in the input circuitbel'ow what it would be if the resistor S! were not present. As the potential across thedevice" ll rises. the potential difference across the resistor 55 also rises but in a direction to oppose the" potential from the device Hi. The result is that the resistor 55, operating in the described manner, maintains linearity of response as between the current in the output circuit and that of the output of the device Ill. The resistor 55 also assists in promptly restoring the output current to zero upon removal of the input signal.

There has already been mentioned the deleterious effects of overloading the amplifier. It has been avoided by the provision of a blockinfllimiter, that is, of means responsive to the output of the amplifier for fixing an upper limit of current which may how in the input circuit. 'As

6" shown, the input-limiting means II comprises a pair of boundary-layer rectifiers, such as copper-oxide or selenium rectifiers connected in parallel relation with each other and for conduction of current in either direction. One side 0! the oppositely-poled and parallel-connected rectifiers Ill is connected by conductor II to one side of the output circuit. Current may fiow through the conductor 1|, the upper rectifier, by way of conductor 12 to resistor 53, and thence by conductor I3 to the other side of the output circuit. with reversed polarity, the current will fiow in the opposite direction in the foregoing circuit. the current then flowing through the lower rectifier. The resistor 52 is also connected in the input circuit and this resistor is included in circuit with a. second pair of rectifiers 14, one side of which is connected by conductor 15 to one side of the output circuit of the second stage of the .amplifier. The other side thereof is connected by conductors l6 and 13 to the junction of resistors 52 and 53. The return circuit from the other side of resistor 52 to the other side of the second stage of the amplifier is by way of conductors SI and 11. Under some circumstances, both of the resistors 52 and 53 may be utilized. For many applications only resistor 52 need be utilized. Thus, the resistor 53 and the means 10 would be omitted. Of course, they would comprise the desired arrangement for a single stage of amplification.

Copper-oxide rectifiers have operating characteristics which make them suitable as output rectifiers for the magnetic amplifiers. Fig. 2 shows characteristic curves for such rectifiers. Specifically, it will be observed that the curve 80, plotted with ohms per square centimeter as ordinates against volts, indicates that so long as the applied voltage is less than approximately one-tenth of 9. volt the resistance of both rectifiers remains 4 very high. As the voltage increases above about one-tenth of a volt, the resistance of one of the rectifiers decreases very rapidly while the resistance of the other rectifier increases, as shown by curves at and 8| respectively. The curve." bears the label Negative polarity" to indicate that the resistance rises when a voltage is applied in a direction opposite to a voltage of "Positive polarity, which results in a reduction in the resistance.

To provide the blocking-limiter, advantage is taken of the combined resistance characteristic of the parallel-connected boundary-layer rectifiers 10 and II. The characteristic curve I! shows that as the voltage rises there is a relatively abrupt decrease in the resistance thereof; for example, at about 0.01 volt. The boundarylayer rectifiers have proved satisfactory. When connected as shown, they exhibit the characteristics shown by curve 82 regardless of polarity of the applied voltage. They exhibit a non-linear and negative resistance-voltage characteristic. Other devices havin similar resistance characteristics may be utilized.

It will now be assumed that the polarity of the current in the input circuit to the first stage of the amplifier is such that current flows from right to left through resistor 53, as viewed in Fig. 1. It will be further assumed that the input current now increases so that the voltage across the output circuit. including the resistors 35 and 31, rises to a value materially in excess of one-hundredth of 1;. volt. The polarity will then be such that the right-hand side of resistor 81 is positive and the left-hand side of resistor 35 is negative. Thereupon, the major current flow is byway oi 7 the upper one of the rectifiers it and through the resistor 53, the current flow through resistor 53 being in the same direction as that of the input current. This introduces a potential difference in the input circuit opposing that oi device It with a resultant limiting action on the input current. As shown by the curve 82, the combined resistance of the input-limiting means it rapidly decreases, as the voltage across the output circuit increases, with the resultant that the current flowing through the resistor tit will rapidly increase.

Similarly, the voltage across the output circuit of the second stage of the amplifier, including the resistors 35a and 316:, will produce substantial current flow through the upper one of the rectifiers l4 and thence through the resistor 52. There is thereby introduced into the input circuit a further potential difference, with a similar limiting action upon the current flowing in the input circuit. It may also be observed the resistor 55 also acts to reduce the input signal and its linear action assists the corrective action of resistors 52 and 53.

Should the current in the input circuit be fiowing in the reverse direction, it will be seen that the polarity of the voltages in the respective output circuits of the first and second stages of the amplifier will be reversed. Accordingly, in the event of overload, current will flow in the opposite direction through the input-limitin means Hi and 14. The action of resistors 52 and 53 will be the same in that they will exert a limiting action upon the flow of current in the input circuit. The provisions, including the resistors 52 and 53, mutually contribute to prevent paralysis or overloading of the amplifier, with consequent zero shift. The provisions cooperate to maintain the amplifier in operative condition for amplification of all currents from the device it which fall within the range in which measurements are of importance.

In the foregoing explanation it will be remem bered the device in was assumed to have a polarity first of one sign and then of the opposite sign. Thus the amplifier may be utilized for amplification of alternating currents of frequencies which are low as compared to the frequency of the source which supplies transformer 32.

While a preferred embodiment of the invention has been described, it will be understood that further modifications may be made without departing from the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

1. The combination with a magnetic amplifier having a current-responsive input circuit and an output circuit, of means non-linearly responsive to the voltage of said output circuit, a resistor connected in series with said input circuit and said means and in shunt relationship to said output circuit.

2. The combination with a magnetic amplifier having a current-responsive input circuit and an output circuit, of means including a resistor connected in series with said input circuit and in shunt relationship to said output circuit for nonlinearly opposing current flow in said input circuit in response to the output of said amplifier.

3. Thecombination with a magnetic amplifier having a current-responsive input circuit and an output circuit, of a resistor in series with said input circuit, and a circuit in series with the output of the amplifier and including said resistor ior resistor opposition to current now in said input developing opposition to current flow in said iii-- put circuit.

4. The combination with a magnetic amplifier having a current-responsive input circuit and an output circuit, of a resistor in series with said input circuit, and an additional circuit connected to said output circuit subjected to the voltage across said output circuit for producing by said circuit.

5. The combination with a magnetic amplifier having independent input and output coils, in current-responsive input circuit connected to said input coils, and a separate output circuit connected to said output coils, of a resistor connected in-series with both said input circuit and said output circuit for varying the magnitude of the current flowing in said input circuit in response to current output of said amplifier.

6. The combination with a magnetic amplifier having an input circuit for flow of current to be amplified by the amplifier and having an output circuit for flow therein of said amplified current electrically separate from said input circuit, of a resistor connected in said input circuit and electrlcal circuit connections to said output circuit for flow through said resistor of current from both of said circuits for reducing the flow of input current.

7. The combination with a magnetic amplifier having a current-responsive input circuit and an output circuit, of a resistor connected in series with said input circuit and in shunt relationship to said output circuit, means in series with said resistor and non-linearly responsive to the output of said amplifier for producing non-linear opposition to current flow in said input circuit. and means including a second resistor connected in series with said input and output circuits for linearly pposing current flow in said input circuit.

- 8. The combination with a magnetic amplifier having a current-responsive input circuit and an output circuit, of at least two resistors connected in series with said input circuit, and means connecting one of said resistors in parallel relationship with said output circuit and for connecting the other of said resistors in series with said output circuit for controlling the magnitude of current flowing in said input circuit.

9. The combination with a magnetic amplifier having separate input and output coils, an input circuit connected to at least one input coil and an output-circuit connected to at least one output coil, of a resistor included in said input circuit and operable by the output of the amplifier for eil'ectively varying the resistance of said input circuit as a function of the output of said said resistor in series with said input circuit and in shunt relationship to said loutput circuit, said means including boundary-layer rectliiers. at least one of which is connected for flow of current in one direction and at least one other of which is connected for flow of current'in the opposite direction for non-linearly opposing current fiow in either direction in said input circuit to limit the magnitude thereof.

12. The combination set forth in claim 11 in which said input circuit includes a second resistor connected in series with both of said input and output circuits for fiow of output current therethrough in the same direction as the fiow of input current therethrough.

13. The combination with a magnetic amplifier having a current-responsive input circuit, an output circuit, and a plurality of stages of amplification therebetween, of resistors in series with said input circuit, means including at least a boundary-layer rectifier for connecting one resistor in shunt relationship to the output circuit of one of the stages of amplification, and means including a boundary-layer rectifier for connecting another of said resistors in shunt relationship to said output circuit of the last stage of amplification for non-linearly opposing current flow in said input circuit to limit the magnitude thereof.

14. The combination set forth in claim 13 in which a third resistor is connected in series with said input and output circuits for linearly opposing current flow in said input circuit.

15. In a magnetic amplifier having an input circuit which includes a current-carrying coil and a voltage output circuit, the combination of means for applying to said input circuit signal currents which may vary in magnitude from zero and which may flow in either direction, and means for producing a potential in said output circuit related to the magnitude of said input current and which reverses upon reversal of current flow in said input circuit.

16. In a magnetic amplifier having an input circuit and an output circuit, the combination of a pair of saturating reactors having output windings, biasing windings energized by unidirectional current and input windings, means connecting said input windings for flow of input current therethrough respectively to produce a magnetomotive force one of which aids and the other of I 10 which opposes that produced by the associated biasing winding, and means including resistors respectively connected to said output windings and across said output circuit to produce a potential which reverses upon reversal of the flow of current in said input circuit.

17. The combination set forth in claim 16 in which said output potential is applied to a second stage of amplification similar to said first stage as defined by claim 16, and in which said output potential of said first stage produces current flow through the input windings of said second stage.

18. In a magnetic amplifier having an input circuit and an output circuit, the combination of means for applying to said input circuit signal currents which may vary in magnitude from zero and which may flow in either direction, means for producing a potential in said output circuit related to the magnitude of said input current and which reverses upon reversal of current flow in said input circuit, a resistor, means connecting said resistor in series-circuit relation with said input circuit, and means connecting said resistor to said output circuit for flow of output current through said resistor in the same direction as said input current, the direction of fiow of said output current through said resistor reversing with reversal of flow of said input current.

19. The combination with a. magnetic amplifier having a current-responsive input circuit and an output circuit, of a resistor in series with said input circuit and means for opposing current fiow in said input circuit comprising an additional circuit including said resistor for flow of current therethrough of magnitude related to the output current in said output circuit.

ALAN S. FITZ GERALD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,164,383 Burton July 4, 1939 2,388,070 Middel Oct. 30, 1945 

